The propagation of a light beam can be described adequately by four characteristic parameters, that is by the spatial offset of the light beam from the optical axis of the optical system, characterised by two coordinates which come out by a parallel shift of the light beam to the optical axis in a plane perpendicular to the optical axis, the angle under which the light beam itself or a line laid through the light beam cuts the optical axis after the parallel shift (elimination of the offset), as well an angle which describes the spatial position of the plane, with respect to a fixed coordinate system, which is defined by two lines each of which running through the light beam and the optical axis.
In the U.S. Pat. No. 5,206,766 a method and a device is disclosed which is used for the adjustment of a laser diode in an optical scanner. For the exact adjustment, in front of the scanner an opaque disc with an aim mark is installed. The exact adjustment of the laser diode is achieved when the laser beam meets the aim mark
The U.S. Pat. No. 5,717,666 shows an adjustment apparatus for an optical means, to determine and equalise the offset between the central axis of a lens and the optical axis of a laser beam. The apparatus discloses a first and a second photo detector. The first photo detector is movable along the central axis of the lens to thus determine the offset. With the second photo detector the distribution of the laser beam is determined. Further, a beam splitter is provided which directs the laser beam onto both photo detectors. A mechanism moves the laser light source on the basis of the signals obtained from both photo detectors to adjust the laser light source accordingly.